The present invention relates to a method for forming an electrostatic image in which an exposure by activation light is effected prior to an imagewise exposure on an electrophotographic photoreceptor comprising a conductive support and an photosensitive layer supported thereon which includes a resin and fine photoconductive powder dispersed therein, and further relates to an electrophotographic photoreceptor suitably adapted for use in this method.
In an electrophotography in the prior art, an electrophotographic photoreceptor is provided with photosensitivity by charging by means of known methods, and then directly formed thereon with an electrostatic image by effecting an imagewise exposure. Alternatively, the electrostatic image is electrostatically transferred on a sheet of transfer paper, then developed by means of a dry or wet developer and fixed to produce a visible image. In the recent electrophotography, on the other hand, the electrostatic image formed on the electrophotographic photoreceptor is developed by a developer to produce a powdered image, which is electrostatically transferred on a sheet of transfer paper and then fixed to form the visible image.
To produce the visible image with the aid of such electrophotography, it is first of all necessary to form an electrostatic image which has excellent characteristics with respect to charging, dark decay and light decay.
Further, an electrophotographic photoreceptor, particularly for use in an electrophotographic copying machine of the repeated transfer type, is required to have characteristics in which it is free from wear or degradation without any reduction in quality of the image, even after the repetitive use thereof for a great number of copies.
The photoconductive material preferably used in electrophotography is, for example, an inorganic photoconductive material such as selenium, zinc oxide, zinc sulphide, cadmium sulphide, cadmium selenide, cadmium sulphide selenide, mercury sulphide, titanium oxide or lead oxide; a photoconductive pigment such as metalfree phthalocyanine, copper phthalocyanine, cobalt phthalocyanine, nickel phthalocyanine, zinc phthalocyanine or lead phthalocyanine; or an organic photoconductive material such as poly-N-vinylcarbazole, anthracene or triallylamine derivatives. An inorganic photoconductive powder and an photoconductive pigment are particularly suitable for an electrophotographic photoreceptor containing a binder resin and a powdered photoconductive material, i.e., photoconductive powder dispersed therein. The photoconductive powder usually used in electrophotography is more than 0.3 micron in average granularity, and thus relatively large. A photoreceptor is further preferably used which contains an amount of resin relative to the photoconductive powder which is as small as possible in order to provide an efficient photoconductivity in the photoconductive powder. In other words, the electrophotographic photoreceptor contains a small amount of resin and relatively large-granulated photoconductive powder dispersed therein, because a photoreceptor with a rapid light decay is required, in view of the fact that high-speed reproduction capable of reproducing a great number of copies in a short time is commercially demanded, because of a tendency to prefer reproduction by means of an electrophotographic copying machine of the repeated transfer type. Such an electrophotographic photoreceptor indeed has a rapid light decay, but poor characteristics with respect to charging, dark decay, image quality and wear resistance.
On the other hand, many efforts have been made to use an electrophotographic photoreceptor containing small-granulated photoconductive powder. Such a photoreceptor has excellent characteristics with respect to charging, dark decay, image quality and wear resistance, but is disadvantageously very poor with respect to light decay. To overcome the drawbacks, proposals have been made in which photoconductive powder having the large granularity is suitably mixed with photoconductive powder having a small granularity, or a photosensitive layer dispersively containing photoconductive powder of large granularity is combined in overlaid relationship with a photosensitive layer dispersively containing photoconductive powder of the small granularity, thereby providing a laminated electrophotographic photoreceptor. These proposals, however, do not produce a superimposed effect of excellent properties, but only average properties. A protective layer is further often provided to protect the surface of the electrophotographic photoreceptor, because it suffers from an electric impact or mechanical wear, in view of the fact that little resin is contained therein. This, on the contrary, causes the degradation of the light decay property and an increase in residual charges, with the result that a practically preferable electrophotographic photoreceptor is not achieved.
In published examined Japanese patent applications Sho 42-5912 and Sho 51-39538 or published unexamined Japanese patent application Sho 48-54946 for example, there is, disclosed a method in which an overall exposure is performed on the electrophotographic photoreceptor prior to the charging and imagewise exposure steps to improve the wear resistance, i.e., one of the properties, of the electrophotographic photoreceptor used in an electrophotographic copying machine of the repeated transfer type. However, such a method is intended to weaken the reduction in charge potential and the increase in dark decay caused by the repetition of the charging and exposure in the electrophotographic photoreceptor, and not to provide a positive improvement in properties of the light decay and image quality.
Further a method is known involving the overall irradiation on the residual toner image, by means of a cleaning lamp for easy cleaning, prior to the removal of the toner by means of a cleaning brush and a method of making an overall exposure, prior to the charging step to restore a memory in the electrophotographic photoreceptor. This method is particularly adapted for use in an electrophotographic copying machine of the repeated transfer type in which the electrophotographic photoreceptor is subjected to the charging and exposure steps to prepare an electrostatic image, which is developed by the developer to produce a toner image transferred on a paper with these steps repeated to reproduce a great number of images. This method is also primarily intended to clean the toner image, and not to improve various characteristics, particularly with respect to the light decay in the electrophotographic photoreceptor.